MOTIONS OF INSECTS. 319 



inwards, as De Geer thinks, and then pushing it with 

 force along the plane of position^. — There is a tribe of 

 minute insects amongst the Aptera, found often under 

 bark, sometimes" in the water, and in various other 

 situations, which Linne has named Podura, a term 

 implying that they have a leg in their tail. This is 

 literally the fact. For the tail, or anal extremity, of 

 these insects is furnished with an in flexed fork'', which, 

 though usually bent under the body, they have the 

 power of unbending ; during which action, the forked 

 spring, pushing powerfully against the plane of posi- 

 tion, enables the animal to leap sometimes two or three 

 inches. What is more remarkable, these little ani- 

 mals are by this organ even empowered to leap upon 

 water. There is a minute black species (P. aquatica, L.) 

 which in the spring is often seen floating on that con- 

 tained in ruts, hollows, or even ditches, and in such 

 infinite numbers as to resemble gunpowder strewed 

 upon the surface. When disturbed, these black grains 

 are seen to skip about as if ignited, jumping with as 

 much ease as if the fluid was a solid plane, that resists 

 their pressure. — The insects of another genus — sepa- 

 rated from Podura by Latreiile under the name of 

 Smbithurus — have also an anal spring, which when 

 bent under the body nearly reaches the head. These, 

 which are of a more globose form than Podura^ are so 

 excessively agile that it is almost impossible to take 

 them. Pressing their spring against the surface on 

 which they stand, and unbending it with force, they are 

 out of your reach before your linger can come near 

 thorn. One of them, S. fuscus, besides the caudal 



' ii, 910. " Plate XY. 1'\g. 10. 



